Hitherto, for improving the impact strength of a thermoplastic resin, a technique of modifying the resin by rubber has been established.
For example, there are an ABS resin obtained by reinforcing a styrene-acrylonitrile resin (AS resin) with a butadiene rubber and an AAS resin obtained by reinforcing as AS resin with an acrylic rubber.
As such a base rubber component, a silicone rubber is considered but when polyorganosiloxane is simply compounded with a thermoplastic resin, the compounded product is insufficient in impact resistance since these components are inferior in compatibility with each other. Thus, a technique of grafting a vinyl monomer on a rubber as in an ABS resin is required, but a polyorganosiloxane is generally poor in a reactivity with a vinyl monomer and hence it is not easy to form a graft polymer of a polyorganosiloxane. Thus, for forming the graft copolymers of this kind, several methods are proposed.
For example, it is proposed in JP-A-50-109282 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") that a graft copolymer having an improved impact strength is obtained by polymerizing a vinyl monomer in the presence of a polyorganosiloxane having a vinyl group or an allyl group.
A method of improving the impact strength of a vinyl polymer by using a polyorganosiloxane having a mercapto group in place of the polyorganosiloxane having a vinyl group or an allyl group is also proposed in JP-A-52-130885.
Furthermore, it is proposed in JP-A-60-252613, JP-A-61-106614, and JP-A-61-136510 that a graft copolymer having an excellent impact strength is obtained with a high graft efficiency by polymerizing a vinyl monomer in an emulsion of a polyorganosiloxane having an acryl group or a methacryl group.
However, when the polyorganosiloxane having a vinyl group or an allyl group or the polyorganosiloxane having a mercapto group is used, the polyorganosiloxane is poor in a reactivity with a vinyl monomer. Accordingly, an apparent graft ratio calculated from an amount of a gel formed, that is, a ratio of the grafted vinyl polymer to the amount of the polyorganosiloxane, is small.
Therefore, an interface adhesive strength between such a polyorganosiloxane and a vinyl polymer is low, thereby causing a remarkable delamination, whereby there is a problem that a graft copolymer having good appearance and a sufficient impact strength cannot be obtained.
Also, when the polyorganosiloxane having an acryl group or a methacryl group is used, a graft copolymer having an improved impact strength is obtained but the heat stability thereof is poor and the graft ratio is greatly lowered by the high-temperature molding and the retention during molding. Consequently, the impact strength, appearance, and luster of the moldings are greatly reduced.